Method of coloring copper and copper alloys



Patented Mar. 13, 1934 UNITED STATES METHOD OF COLORING COPPER ANDCOPPER ALLOYS John It. Freeman, in, Cheshire, Kirby, West Cheshire,Conn.,

and Philip H. assignors, by

mesne assignments, to Copper and Brass Research Association, New York,N. Y., a voluntary association of New York No Drawing. Application January.-20, 1932, Serial No. 587,754

9 Claims.

This invention relates to the coloring of copper and copper alloys, andhas for its object the production on copper and alloys. of copper of .a

pleasing color or patina, in particular, a green coloration.

Copper and copper alloys, such as copper roofs and ornamental bronzesare widely used. It is known that copper and its alloys when exposed tothe atmosphere will in time due to natural weathering take on a pleasinggreen coloration- The rate at which copper and copper alloys 25 take onor acquire a natural green coloration or patina varies greatly accordingto atmospheric conditions and requires in any case a period of years todevelop.

Numerous previous attempts have been made 36) to rapidly develop thepatina or green color by treatment with various chemicals and mixturesof chemicals. Green coloration may be ,developed quite readily by suchtreatment but we have found that coloring so produced by any 33 methodknown to us is not permanent, the coloring washing off gradually in rainstorms giving the surfaces that have been treated a streaked andunsightly appearance.

We have found by chemical analysis of .sam-,

ples of patina scraped from samples of old copper roofs taken from therailway station at Providence, R. I., and from the Yale UniversityLibrary at New Haven, Conn., and other samples that the patina or greencoloration that develops on copper and copper alloys, such for exampleas alloys of copper and zinc, by natural weathering is principally thechemical, basic sulfate of copper. This compound when formed by naturalweathering on the surface of copper or copper 59 alloys is adherent andis insoluble in water and so is permanent. The reason previous methodsof producing patina on copper have failed is that the chemical formulaand solutions were not of such nature as to cause to be formed on thecopper the basic sulfate such as is formed by natural weathering,it-having always been believed by previous workers that the patina was abasic car bonate of copper which they therefore attempted to reproduce.

What we have discovered therefore and wish to secure by Letters Patentis the production by suitable treatment on the surface of copper of anadherent and permanent patina, or more particularly an adherent andpermanent green coloration which is of similar chemical composition tothe basic sulfate of copper that develops by natural weathering afterexposure to the atmosphere for a period of years. The chemical compoundknown as basic sulfate of copper that is formed by natural weathering oncopper or copw per alloy surfaces is of a complex nature and may existin different forms, but corresponds in general in chemical constitutionto the formula CuSO4.3Cu(OH2). Similarly the patina that we produce byaccelerated means may exist in different forms but is essentially of thesame constitution as the patina that forms by natural weather- We havefound that to obtain the most satisfactory patina or green coloration oncopper and its alloys, such for example as the brasses and bronzes,although we are not limited to these alloys, having suitable adherenceand uniformity, that it is best produced by alternate immersion of thearticle of copper or copper'alloy to be colored in a solution ofammonium sulfate. Sim-= ilar coloring may be obtained by other meanssuch as repeated or continuous spraying or swabbing with a solution ofammonium sulfate or any means by which the copper or copper alloysurface is made to come in contact with a solution of ammonium sulfateand oxygen either as such or as it exists in the atmosphere.

In our preferred method the article or articles to be colored are dippedin a solution of ammonium sulfate for sufiicient time to permit thesolution to moisten thoroughly all parts of the surface to be coloredand then are withdrawn from the solution and permitted to remain incontact with theair to permit oxidation attack by the oxygen of the air.I

This cycle of alternate dipping into the solution and oxidation by theatmosphere is repeated until the desired coloring or patina is obtained.

The solution 1 is prepared by dissolving ammonium sulfate in water. Apure grade of ammonium sulfate may be desirable for some purposes butfor reasons of economy less pure grades such as used for fertilizer maybe used and satisfactory results obtained. We prefer to use a incconcentration of about 5 to 10% ammonium sulfate in water but diluted orhighly concentrated solutions or that is solutions of any concentrationmay be used. We find it of advantage to heat the solution to from about50 C.- C. In order to hasten the process but it maybe done with thesolution cold. The process may be hastened by maintaining the atmosphereto which the articles are subjected during the oxidation phase of. theprocess in a highly humid condition but this is not necessary toobtaingood results. An atmosphere of pure oxygen instead of the normalatmosphere containing only about 20% oxygen may be used.

The length of time that the article is in the solution and in the airwill depend upon comvplexity of design and size of the part to becolored and other factors such as temperature, concentration of solutionand humidity of the oxidizing atmosphere.

We have found a period of about 4 seconds in a 10% solution of ammoniumsulfate at 50 C. and 2 minutes in the air to give satisfactory results.

Under some conditions of treatment it is found that the color obtainedis a darker green than that which develops by natural weathering. Thisdarker shade is due probably to the retention in the patina formed ofcertain copper-ammonium compounds. These salts or impurities may beremoved and the more normal shade of green obtained by subjecting thearticle to a hot humid atmosphere for such length of time as is foundnecessary to obtain the desired shade, such for example as'an atmosphereof a temperature above 30 C. and more or less saturated with moisture. IHaving thus set forth the nature of our invention, what we claim is:

1. The method of producing on the surface of copper and copper alloys anadherent green coloration of patina similar in character to that whichforms due to natural weathering after a period of I time which comprisestreating the copper or copper alloy surfaces with a solution of ammoniumsulfate and thereafter exposing to oxygen.

2. The method of producing'on the surface of copper and copper alloys ofan adherent green coloration or patina similar in character with thatwhich forms due to natural weathering after a period of time whichcomprises alternately immersing the copper or copper alloy surfaces in asolution of ammonium sulfate and exposing them to an oxidizingatmosphere.

3. A method of producing on the surface of copper or copper alloys 9.green coloration or patina which comprises alternately moistening the s'faces of the copper or copper alloy article to be colored by an ammoniumsulfate solution and subjecting it to the action of oxygen.

4'. A method of producing on the surface of copper'and copper alloys anadherent green coloration or patina which comprises alternately dippingthe surfaces to be colored in a 2 to 15% water solution of ammoniumsulfate and then subjecting to oxidizingaction of the atmosphere.

5. A method of producing on the surface of copper or copper alloys agreen coloration or patina which comprises alternately immersing thesurfaces to be colored in a solution, maintained at an elevatedtemperature, containing about 10% ammonium sulfate for such period oftime as to permit the surfaces to be wet by the solution and thenwithdrawing the surfaces from the solution and letting them be exposedto a humid atmosphere for sufiicient length of time to permit oxidizingaction to occur, the cycle of alternate wetting by the solutionandoxidation by the atmosphere being repeated a suflicient number oftimes to produce the depth of coloring desired.

6. The method of producing on the surface of copper and copper alloys ofan adherent green coloration or patina similar in character with thatwhich forms due to natural weathering after a period of time whichcomprises wetting the copper or copper-alloy surfaces with a solution ofammonium sulfate while exposed to an oxidizing atmosphere.

7. The method of producing by accelerated means on the surface of copperand copper alloys of an adherent green coloration or patina similar incharacter with that which forms due to natural weathering after a periodof time Which comprises alternately wetting the copper or copper alloysurface with a water solution of ammonium sulfate and letting it remainin contact with the air to permit oxidizing action to occur.

8. A method of producing on the surface of copper or copper alloys anadherent green coloration or patina which comprises alternatelymoistening the surface to be colored with a 2 to 15% water solution ofammonium sulfate and subjecting to the oxidizing action of theatmosphere.

9. A method of producing on the surface of copper and copper alloys anadherent green coloration or patina which comprises alternately dippingthe surfaces to be colored in a 2 to 15% water solution of ammoniumsulfate and then subjecting to the action of oxygen.

JOHN R. FREEMAN, JR. PHILIP H. KIRBY.

